The Angiopoietin-Tie receptor-ligand system plays a central role in both developmental as well as tumor-induced angiogenesis. Its activation during tumor progression highlights the importance of understanding the complexities of this unique signaling pathway from both a basic as well as therapeutic perspective. In agreement, it has recently been proposed that therapies targeting the tumor vasculature may be less toxic and more efficacious than conventional cancer treatments. Therefore, to gain a more accurate molecular understanding of the events surrounding angiopoietin signaling and to develop the potential for identifying novel therapeutic strategies, we have set forth the following goals for this proposal: i) determine the structural principles behind receptor clustering and activation by examining a full length angiopoietin-Tie2 signaling complex, ii) Examine the functional role for Tie1 in angiopoietin signaling, specifically testing the hypothesis that Tie1 functions as a co-receptor for Tie2 signaling and that complementary electrostatic surfaces identified in our Tie2 structure, and Tie1 homology model, allow them to associate, iii) determine the role of alpha5beta1 integrin in angiopoietin signaling by biochemically characterizing the interactions of the angiopoietins and Tie2 with the alpha5beta1 integrin receptor, and iv) identify chemical tools, including small peptides, that will inhibit the formation and signaling of an angiopoietin-Tie2 complex, a prerequisite to the development of Tie2-based anti-angiogenesis therapeutics. The angiopoietins and Tie2 play central roles both in developmental and tumor-induced angiogenesis. Indeed, Tie2 activation during tumor growth, development, and metastasis suggests that the ability to modulate the receptor- ligand interactions would have important medical applications. We propose to extend our present understanding of angiopoietin-Tie signaling by using the tools of X-ray crystallography combined with other biochemical and biophysical techniques to examine how these molecules function at the molecular level.